It is well known to construct measuring probes for fluid conductivity measurement by placing a first magnetic field generating toroid coil in an arrangement relative to a second, magnetic field measuring toroid coil where the conductivity of a fluid magnetically couples the coils. Typical devices known in the art also include placement of a signal source coil and a receiving coil on separate fluid paths as in U.S. Pat. No. 2,709,785 to Fielden.
The principal of fluid measurement by magnetic measurements is discussed in U.S. Pat. Nos. 2,642,057, 3,806,798 and 4,220,920.
An Industrial Instruments Inc. Catalog 27, believed copyrighted in 1963, discusses solution conductivity measurement and theory. This catalog is believed to have been classified by the United States Patent Office in class 324 subclass 439.
The conventional configuration of the toroid coils in the prior art has been the placement of a first toroid coil apart from a second one, where one carries a current and the other is used for measurement.
The conductivity of the solution surrounding the cell and within the cylindrical bore provides link-coupling between the toroid windings for the transfer of electromagnetic energy between the two toroid windings. There is a direct ratio between the conductivity of the fluid being measured and the amount of energy transferred to the receiving toroid coil with a given cell factor (constant). The cell factor is governed by the ratio of the length of the cell to the cross-sectional area of the bore. Therefore, a doubling of the length will double the cell factor.
This type of cell has been used extensively in the measurement of fluids in the order of 10 (100 microsiemens/cm) to 2000 (500 microsiemens/cm) ohms. In order to make measurements below 500 microsiemens/cm the cell must be made with a very low cell factor, and a high-gain electronic circuit must be used to detect the signal. Under these circumstances a large noise factor is often present in the measurement. This noise comes from the solution being measured and from the high-gain electronic circuit which must be used. The presence of this noise makes it very difficult to make accurate and reliable measurements.